Semiconductor laser oscillators (i.e., laser light sources, hereinafter lasers) and semiconductor laser amplifiers (i.e., hereinafter amplifiers) were first developed in the 1960s. Such laser oscillators and amplifiers offered the obvious advantage of extremely small size over the other types of lasers. (A typical semiconductor amplifier may be on the order of few hundred micrometers long). These first semiconductor lasers were fabricated of a single type of semiconductor.
A modern semiconductor laser oscillator or amplifier typically comprises a semiconductor heterostructure, that is, it is made from more than one semiconductor material such as gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs). Semiconductor oscillators and amplifiers are made from a combination of semiconductor materials which have different bandgap energies in order to achieve electrical carrier confinement as well as different optical indices of refraction in order to achieve optical confinement.
Many approaches have been proposed to achieve the goal of high-power, continuous-wave operation with a single-lobed spatial mode output from semiconductor lasers or semiconductor amplifiers. One such approach is to employ a laterally tapered gain region.
An exemplary double heterostructure tapered amplifier 10 of the prior art is illustrated in FIGS. 1 and 2 and comprises three layers of semiconductor material; 1) a p-type material 12 with a relatively high bandgap, such as AlGaAs, 2) an n-type material 14 with relatively high bandgap, which may also be AlGaAs, and 3) a relatively low bandgap p-type material 16, such as GaAs, sandwiched between the other two layers and comprising the active region of the amplifier. More complex structures are also known. The active layer, instead of comprising a single layer of GaAs, may comprise a more complex multiple layer sequence. Layer sequences are known which provide higher gain and efficiency than the single layer GaAs active layer. For instance, a series of very thin layers into which carriers are injected may comprise the active region. Quantum size effects and higher carrier densities provide more gain in such structures. However, this is typically achieved at the cost of lower optical confinement. The lower optical confinement of such quantum wells may be offset by incorporating adjacent layers of intermediate refractive index. Single or multiple quantum wells with a variety of optical confinement structures generally provide superior performance to the single, thick active layer type devices. Performance can also be improved by introducing lattice strain into the quantum wells with particular alloy selections.
In general, any direct-band-gap semiconductor ternary or quaternary alloy system such as AlGaAs or InGaAsP whose various alloys can be selected to have lattice constants close to that of the growth substrate crystal can be used for laser amplifiers or oscillators.
An index-guided, linearly tapered gain region is formed in the active layer 16 having lateral boundaries illustrated by phantom lines 17 and 19 in FIG. 1. Commonly, the gain region is formed by etching through the active layer 16 and regrowing another semiconductor layer in the etched region. A metal contact pad 18 is placed in electrical contact with the top surface of the top layer commensurate with the underlying gain region. Accordingly, lines 17 and 19 also correspond to the boundaries of the electrical contact that pad 18 makes with the semiconductor. When sufficient current is passed through the metal contact, electrons and holes are injected into the active layer 16 in the gain region from the high bandgap material layers 12 and 14. These electrons and holes are trapped in the potential well created by the low band gap GaAs material. Since the electrons are trapped in the active region 16, they are forced to combine with each other in the GaAs material. Light introduced into this region will be amplified.
Confinement of the light around the GaAs active layer 16 is provided by the wave guide properties of the AlGaAs/GaAs/AlGaAs material structure. The AlGaAs layers have a lower optical index of refraction than that of the GaAs material thus providing total internal reflection of light off of the interfaces 13 and 15 so that most of the light remains within the GaAs layer 16 allowing active layer 16 to act essentially as a waveguiding layer.
In operation, substantially diffraction limited light generated by a low power laser 11 is focused by a lens system 21 on the input facet 20 of tapered amplifier 10. If the beam is allowed to spread naturally without any external interference, the beam will remain diffraction-limited as it spreads, thus leading to the desired single-mode amplified output beam. The expansion of the beam reduces the possibility of optical damage at the output facet 22 because the power in the beam is more spread out.
Due to the high gain achieved by semiconductor optical amplifiers, such amplifiers are easily susceptible to self-oscillation. Self-oscillation occurs when a small portion of the light striking the output facet is reflected back into the semiconductor medium. This reflected light is further amplified and a portion reflected again off of the input facet. If the total round trip product of amplification gain and reflection loss reaches unity, self-oscillation occurs. In this case, self-oscillation will build up from internal spontaneous emission, even in the absence of externally injected light.
In the case of semiconductor oscillators, as opposed to amplifiers, self-oscillation is necessary and, in fact, constitutes laser action. However, self-oscillation is undesirable in semiconductor amplifiers because it interferes with the amplification of the input light and may degrade the output mode quality as well as reduce gain.
This problem can be partially alleviated by using anti-reflection coatings on the input and output facets 20 and 22. However, a sufficiently small residual reflectivity is often difficult to achieve in practice and, in fact, may be impossible to incorporate in certain monolithic implementations where a semiconductor master oscillator laser and a semiconductor amplifier are integrated on the same chip. The problem is particularly severe in long amplifiers where the larger gains achieved can easily overcome very small reflection coefficients.
Examples of the state of the art of tapered semiconductor laser amplifiers are Bendelli, G., Komori, K., Arai, S., and Suematsu, Y., A New Structure For High-Power TW-SLA, IEEE Photonics Technology Letters, Vol. 3, No. 1, January, 1991, which discloses an exponentially tapered semiconductor laser amplifier having a high refractive index gradient at the boundaries of the tapered gain region; and Yazaki, P., Komori, K., Bendelli, G., Arai, S., and Suematsu, Y., A GaInAsP/InP Tapered Waveguide Semiconductor Laser Amplifier Integrated with a 1.5 .mu.m Distributed Feedback Laser, IEEE Transactions Photonics Technology Letters, Vol. 3, No. 12, December, 1991, which discloses an exponentially tapered waveguide semiconductor laser amplifier monolithically constructed with a distributed feedback laser. The Yazaki et al. device also has a high refractive index gradient at the boundaries of the gain region.
Accordingly, it is an object of the present invention to provide an improved semiconductor laser amplifier.
It is a further object of the present invention to provide a tapered semiconductor amplifier with increased ability to suppress self-oscillation.
It is another object of the present invention to provide an improved high power, tapered semiconductor laser amplifier with improved spatial mode quality.
It is yet another object of the present invention to provide an improved semiconductor laser oscillator.
It is one more object of the present invention to provide a semiconductor gain structure that can be used as a laser oscillator or as a laser amplifier depending on the reflectivity of the input facet of the structure.